1. Field of the Invention
The present invention relates to an electrophotographic photosensitive member and, more particularly, to an electrophotographic photosensitive member having more uniformity in photosensitivity.
2. Description of the Prior Art
Electrophotographic photosensitive members have been conventionally manufactured in such a manner that a resin layer or a photosensitive layer is formed on a substrate by coating. Among several coating methods, a dip-coating method wherein a substrate is dipped in a coating solution and then withdrawn so as to coat a coating material thereon is preferably used since the coating material can be evenly coated on a substrate of any shape.
In this case, the thickness of a coating film is determined by the concentration of the coating material and the withdrawing velocity with respect to the coating material. It is known that the film thickness is increased in accordance with an increase in the coating material concentration.
However, when the withdrawing velocity is high, a sagging phenomenon occurs while the coating film is being dried and solidified. Thus, the thickness of an upper portion of the coating film is relatively small and that of a lower portion thereof is relatively large. Particularly, when the concentration of the coating solution is low and the viscosity thereof is high, since the amount of solvent used is increased, the sagging phenomenon tends to easily occur.
This tendency becomes especially pronounced in the case of coating a charge transport layer in a multifunction type electrophotographic photosensitive member having a charge generation layer and a charge transport layer. Generally, a charge transport layer is coated using a coating solution such that an electron donor material or an electron acceptor material is dissolved in a solvent together with a film forming resin. However, since the electron donor material or the electron acceptor material, particularly hydrazone compounds, styryl compounds and pyrazoline compounds, has a low solubility in a solvent, a large amount of solvent must be used. For this reason, the coating solution of the charge transport layer has a low concentration, and since the coating solution must be coated to a proper thickness, the viscosity thereof is increased. When a coating solution having a high solvent concentration is coated on a member by the dip-coating method, after the withdrawal process, since the film is dried slowly, the coating film runs downward before it solidifies. Such a phenomenon causes the film to be irregular in thickness, as shown in FIG. 1. Conventionally, in order to reduce such an irregularity in film thickness, the withdrawing velocity is initially set at a high rate, and the rate then is linearly decreased. However, it is difficult to completely eliminate such a sagging irregularity, and this sagging results in an irregular thickness of the coating film.
When the charge transport layer has an irregular film thickness, the charge potential characteristic of the film is uneven. In other words, a thick portion of the charge transport layer has a high charge potential, and a thin portion has a low charge potential. When the exposure amount required for attenuating the charge potential to a constant value (e.g., 150 V) is expressed as a photosensitivity, then because the initial charge potential is high, the attenuation width of the potential must be set to a large value, resulting in poor photosensitivity. For this reason, when the film thickness of the charge transport layer is large, the photosensitivity thereof tends to be poor.
On the other hand, since the photosensitivity also depends upon an amount of charges generated upon exposure, photosensitivity tends to be good in proportion to the increase in the film thickness of the charge transport layer. Since an irregular coating film coated on the photosensitive member causes the characteristics of the photosensitive member to be markedly unstable, a manufacturing method to produce a photosensitive member having a uniform coating film has long been desired.